E-ELT MAIN STRUCTURE

The telescope structure is an alt-az mount (see Figure 1) and it is
divided into two main parts: the Azimuth Structure and the
Altitude Structure. The main structure, including the hosted units and the
items intended for operational use, weighs about 2800 t.
Two massive cradles provide the
rotation of the altitude axis while the azimuth tracks
allow the rotation of the telescope about the zenith. The
challenge in such a massive design is to provide a stiff enough
interface for the primary mirror segments, while at the same time not
dramatically increasing the weight of the structure or
overcomplicating the support. Scaling the size of M1 to the current dimensions has made it possible
to achieve a lighter, stiffer and more compact telescope structure.

The main structure of the telescope was designed as to comply with
critical performance needs, such as pointing and tracking specifications
of 1 and 0.3-arcseconds, respectively. It is required that the
main structure tracks sufficiently well under wind loads. Another
important source of wavefront error is the movement of the secondary units
with respect to the primary, and the support of the telescope was
designed as to limit both the amplitude
and frequency of the M2 unit deflections.

Azimuth structure

The azimuth structure (see Figure 2) supports the scientific instruments. It
rides on three azimuth rings, of 51.5-m, 34-m and 4.5m diameter.
The two biggest rings
support the main vertical reactions during all operational modes. The radial loads
are supported by the intermediate ring, while the central ring supports part of the
rotating floor.
The azimuth structure is made of hollow members and weighs about 1300 t.

The azimuth structure contains also the two Nasmyth platforms, whose dimensions are
approximately 12 m x 29 m and that weigh about 150 t.
The Nasmyth platforms host the pre-focal stations
and provide stable platforms for the post focal instrumentation.

Altitude structure

The altitude structure (see Figure 3) hosts the
telescope optics. It is made of hollow beams and
is estimated to weigh approximately 1500 t with the hosted units. The major
challenges that have been addressed in the design are the need to keep
the primary mirror segments within a reasonable range from their
prescribed locations and the need to minimize the deflections
of the secondary mirror.

Central tower

As shown in Figure 3, the large central obstruction of the optical design
also provides ample room for the erection of a central tower
(see also Figure 4). This tower supports M3, M4 and M5,
together to the Atmospheric
dispersion compensator (ADC) and the adaptive optics calibration
unit (CU).
The CU creates artificial sources at
the side of the central tower and projects them on a small mirror
positioned close to M4. An optomechanical relay
system inserts the source light beam inside the telescope train when
the CU is in operation.